Natural Frequencies of Axially Loaded Bridge Members

The purpose of this paper is to present some formulas that can be used to check the aerodynamic stability of the more typical bridge members having two axes of symmetry. The principal task consists of finding the natural frequencies of those members whose shear center and centroid coincide and therefore vibrate in uncoupled modes. Oscillation is generally due to vortex shedding which in turn causes an alternating force to act on the member. The frequency of the vortex shedding increases with the wind velocity. When it coincides with the natural frequency of the member at the ..critical.. wind velocity, resonance occurs and the amplitude of vibration may reach its maximum value for a stable member. With further increase in velocity, amplitudes may increase to failure by galloping for an unstable member. It is important to know that vortex shedding causes the member to vibrate in bending in the direction transverse to the wind.